1. Field of the Invention
This invention relates broadly to formation fluid collection. More particularly, this invention relates to a single probe formation tester that permits a relatively quick recovery of formation fluids without contamination caused by borehole fluids.
2. State of the Art
During drilling of a wellbore, a drilling fluid (“mud”) is used to facilitate the drilling process. In order to avoid a blowout of the well, the drilling mud is maintained at a pressure in the wellbore greater than the fluid pressure in the formations surrounding the wellbore. In many instances, the drilling mud is often an oil-based mud (“OBM”). Because of the pressure difference between the wellbore mud and the formations, the drilling fluid penetrates into or invades the formations for varying radial depths (referred to generally as invaded zones) depending upon the types of formation and drilling fluid used. The OBM miscibly mixes with the crude oil, thus making separation of crude oil from any collected samples difficult.
When samples of native fluids are desired after drilling, formation testing tools are used to retrieve the formation fluids from the desired formations or zones of interest. Much time is spent trying to obtain native formation fluids substantially free of mud filtrates, and collect such fluids in one or more chambers associated with the tool. The collected fluids are sometimes optically and/or electrically analyzed downhole, but are also often brought to the surface and analyzed to determine properties of such fluids and to determine the condition of the zones or formations from where such fluids have been collected.
Formation fluid testers utilize fluid sampling probes. The testers typically include a pad that is mechanically pressed against the formation to form a hydraulic seal, and a metal tube or probe which extends through the pad in order to make contact with the formation. The tube is connected to a sample chamber, and a pump is used to lower the pressure at the probe below the pressure of the formation fluids in order to draw the formation fluids through the probe. In some prior art devices, an optical sensor system is utilized to determine when the fluid from the probe consists substantially of formation fluids. Thus, initially, the fluid drawn through the probe is discarded. When the fluid samples prove to be uncontaminated from the OBM, the fluid samples are diverted to the sample chamber so that they can be retrieved and analyzed when the sampling device is recovered from the borehole. However, it has been found that it can take an inordinate of time (e.g., many hours) for an uncontaminated fluid sample to be obtained.
In order to reduce the time is takes to obtain an uncontaminated fluid sample, U.S. Pat. No. 6,301,959 to Garnder et al. proposes the use of a probe system including a hydraulic guard ring probe surrounding an inner probe, with a seal therebetween, and an outer seal between the guard region and the formation. The guard ring is used to isolate the inner probe from the contaminating borehole fluid. The guard ring is provided with its own flow line and sample chamber, separate from the flow line and the sample chamber of the probe tube. By maintaining the pressure in the guard ring probe at or slightly below the pressure in the inner probe tube, according to Garnder et al., most of the fluid drawn into the inner probe tube after a reasonable time will be connate formation fluid.
The Gardner et al. solution suffers from various drawbacks. For example, the use of two seals with the outer guard ring and the inner probe tube is a relatively complex arrangement. In fact, the arrangement with two seals is prone to failure, since, as admitted by Garnder et al., the seals often do not function as intended. In addition, the arrangement of the Garnder et al. invention requires careful control of pressure in the guard and sample lines so as to obtain the full “guard effect”.